This invention generally relates to water jet propulsion systems for propelling boats or other watercraft. In particular, the invention relates to outboard water jet propulsion units comprising an engine and a ducted impeller driven by the engine.
Jet-powered boats can be categorized in part in accordance with the types of propulsion systems used. The powerhead can be mounted either inside the hull or outside the hull. In the latter case, the powerhead is mounted on the transom portion of the boat hull and is detachable. Another type of system, called stern drive system, and sometimes referred to as an inboard-outboard system, utilizes a powerhead mounted inside the hull of the boat with a portion of the drive unit extending through the transom. These systems create thrust through rotation of a ducted impeller, which draws water from ahead and impels the water rearward to propel the boat forward.
To facilitate use of water jet-propelled boats in shallow water, it is known to mount the ducted impeller at an elevation such that the propulsion system does not project below the bottom of the boat hull. This can be accomplished, for example, by installing a duct in the stern of the boat, the duct being arranged to connect one or more inlet holes formed in the bottom of the hull with an outlet hole formed in the transom. The water jet is then installed outside the hull in a position such that the water jet inlet is in fluid communication with the duct outlet at the transom. Alternatively, a water tunnel is formed in the bottom of the hull which is open at the bottom and at the transom. A water jet propulsion system is then mounted to the transom by means of a mounting adapter, the inlet of the propulsion system being in fluid communication with the water tunnel via the adapter.
The use of outboard water jet propulsion units is not new to the marine industry. However, the majority of these applications utilize a centrifugal pump, which allows the propulsion unit to be designed so that it does not extend below the hull bottom. An axial-flow pump is disclosed in U.S. Pat. No. 3,842,787. However, the inlet duct of that system extends below the hull bottom.
There is a need for an axial-flow outboard water jet propulsion unit which does not extend below the hull bottom.
As used herein, the term xe2x80x9caxial-flow pumpxe2x80x9d means a ducted impeller in which the water is impelled in a direction generally parallel to the axis of impeller rotation. This is in contrast to a centrifugal pump, in which water is impelled radially outward in directions perpendicular to the axis of impeller rotation.
The present invention is directed to an outboard water jet propulsion unit comprising an axial-flow pump. Preferably the water jet propulsion unit is designed so that it does not extend below the bottom of the boat hull, but rather is disposed directly behind the hull transom. This is achieved by arranging the impeller shaft generally perpendicular to a generally vertical drive shaft, and by installing the impeller in a flow-through duct which, although placed directly behind the transom, has an inlet for taking in water located directly below the impeller shaft. In the preferred embodiment of the invention, the duct does not extend to a depth lower than the lowest point of the hull bottom. The impeller shaft penetrates the duct wall, while the impeller itself is rotatable inside the duct. The impeller impels water rearward toward a convergent exit nozzle of the duct, sucking in water through the duct intake. The aft opening of the exit nozzle forms the duct discharge outlet.
In accordance with the preferred embodiment of the invention, the propulsion unit comprises: an engine support housing; an engine supported by the engine support housing; a drive shaft having one end coupled to the engine; an inlet housing attached to and located beneath the engine support housing; and an outlet housing attached to the rear of the inlet housing. The inlet housing comprises a flow-through passage having an inlet and an outlet. The outlet housing comprises a flow-through passage in fluid communication with the inlet housing passage. Preferably, the outlet housing passages converges toward an outlet. The inlet and outlet housings form a duct. In addition, the propulsion unit may comprises: an impeller shaft which penetrates a wall of the inlet housing; an impeller mounted to the impeller shaft and rotatable within the inlet housing passage; bearings for rotatably supporting the shafts; and gears for converting rotation of the drive shaft into rotation of the impeller shaft. The impeller shaft is generally perpendicular to the drive shaft, and the inlet of the inlet housing lies underneath the impeller shaft.
In accordance with a further feature of the preferred embodiment, the engine support housing, the inlet housing and the outlet housing each comprise a respective exhaust gas passage. These exhaust gas passages are connected in series so that exhaust gas from the engine is discharged below the waterline.
In accordance with another feature of the preferred embodiment, the gears are housed in a gear housing which is integrally formed with the inlet housing.
In addition, a steering nozzle is pivotably mounted to the exit nozzle. The steering nozzle is pivotable about an axis which is generally perpendicular to the impeller shaft. Because the boat can be steered by turning the steering nozzle, the outboard water jet propulsion unit does not need to be pivotable about a vertical axis, thereby simplifying the mounting of the outboard propulsion unit to the boat hull.